Papermaking is a sophisticated operation involving massive and very expensive machines. These machines are increasingly running at higher speeds, meaning that their overall efficiency must be very high, and in particular, the efficiency of their sub-components must also be very high. The papermaking process requires that water be removed from the initial pulp fiber solution as the paper is formed. The pulp fiber solution, once in the drying section of a papermaking machine, is referred to as the paper web. The paper web is supported as it travels across the machine following a path during which moisture is progressively removed therefrom. The support is provided by endless sheets of porous fabric, felts, wires or other water and gas permeable support means, all of which are generically referred to as the  less than  less than felt or felts greater than  greater than  in the description and appended claims.
The paper web travels from what is referred to as the wet end of the machine to the dry end thereof. In its path, the paper web runs over numerous heated drying cylinders where moisture is evacuated therefrom either by direct evaporation or transfer of moisture to the felts or to the surface of the drying cylinders. A network of ventilator devices are used throughout the drying section in order to inject heated dry air at numerous locations and promote the removal of moisture from the papermaking machine. At the end, the machine outputs the resulting paper, which is then generally reeled to be shipped elsewhere.
Papermaking machines can be built according to numerous possible configurations. One configuration in particular is the twin-wire draw, where the papermaking machine comprises two superposed rows of axially-parallel and horizontally disposed heated drying cylinders. The paper web runs in a serpentine or zigzag path where it defines loops by alternating between the two rows of drying cylinders as it advances along the drying section. The paper web is being supported in most of its path with the assistance of the felts. There is generally one felt for each row of drying cylinders. Each felt presses the paper web on a portion of the surface of the drying cylinders of the corresponding row. Each felt also runs over a felt roll between each pair of adjacent drying cylinders of a same corresponding row. The felt rolls are located deep in the space between the two adjacent drying cylinders. This configuration allows to maintain the paper web in supporting contact with the felt as long as possible. The felt rolls essentially redirect a felt to the next drying cylinder of the same row.
It should be noted at this point that the terms  less than  less than roll or rolls greater than  greater than  and  less than  less than cylinder or cylinders greater than  greater than  are synonyms since both are elongated member with a circular cross-section, the only distinctions in the present context being that the drying cylinders are generally much larger in diameter than the felt rolls and are heated by appropriate means that are well known in the art. The segregated use of the terms in the text is only for the purpose of clarity. The  less than  less than cylinder or cylinders greater than  greater than  are sometimes referred to as  less than  less than drum or drums greater than  greater than  in other documents.
Traditional drying cylinders and felt roll arrangements feature the felt roll being positioned intermediate the axis of rotation of adjacent drying cylinders, thus halfway between two adjacent drying cylinders. Inherent in these arrangements is the fact that the paper web is repetitively unsupported wherever the felt separates from the paper web to pass over a felt roll.
In order to increase the speed of papermaking machines, it was desirable to reduce the length of unsupported paper web because of the risks of rupture thereof, particularly near the wet end where the paper web is weaker. One solution to this problem was to move the axis of rotation of the felt rolls backwards, more particularly towards the wet end of the machine, as illustrated in FIG. 1. This offset configuration has resulted in the felts and the paper web being in contact longer, thus minimizing the length of unsupported paper web as it travels from one drying cylinder to another.
The offset configuration of the felt rolls has also resulted in creating what is known as offset pockets. In FIG. 1, the pocket spaces (12) are identified as hatched areas. Each pocket space (12) is situated between three successive drying cylinders (20) over which consecutively run a paper web (14). There are thus many offset pockets (12) since there are many groups of three successive drying cylinders (20. A pocket space (12) may be roughly defined as the space limited by a felt (16) between two successive drying cylinders (20) of a same row, a first draw of paper web (14) from a first drying cylinder (20) to the next drying cylinder (20) of the other row, a second draw of paper web (14) between that second drying cylinder (20) and a third successive drying cylinder (20) on the same row as the first one, and the free surface the second drying cylinder (20). The pocket spaces (12) are only open at each side of the machine. It should be noted that the pocket ventilators have been omitted from FIG. 1 for to simplify the drawing.
Unfortunately, ventilating offset pocket spaces is more difficult than in symmetrical pockets spaces found in traditional configurations. This results from the reduction of the length of felt which does not support the paper web on the side upstream of the felt roll. Since the felt is permeable to air and the paper web is not, the conventional dry heated air ventilators which were hitherto provided to ventilate the felt cannot be used the same way since air cannot be blown through the felt when it is supporting the paper web. Moreover, an offset pocket is often too small for receiving a ventilating device therein. All of this has resulted in decreased air flow into offset pocket spaces, thus a decrease in the efficiency of the ventilation.
An example of a pocket ventilator previously known in the art is disclosed in U.S. Pat. No. 5,074,278 to Turcotte and issued Dec. 24, 1991. It illustrates a traditional symmetrical arrangement of pocket spaces. This patent is hereby incorporated by reference.
The geometry of a pocket and the natural air currents generated in the pocket space by the moving paper web and felt, as well as the rotation of the drying cylinders and the felt rolls, are key factors which increase the difficulty in ventilating a pocket space. Air introduced in a pocket space has a natural tendency to follow the movement of the felt and also tends to be trapped in a cleft defined by the felt and a downstream drying cylinder. This cleft, known as the closing nip, is a zone of positive air pressure where air tends to flow through the felt to evacuate the pocket space. Furthermore, a zone of negative air pressure is created in a cleft defined where the paper web leaves the felt roll on its way to the next drying cylinder. Some air evacuates the pocket space to satisfy the negative pressure created therein at the opening nip. However, these natural air currents do not provide an adequate ventilation of the pocket space as they do not effectively sweep the pocket space. As a result, air becomes entrapped therein, thereby increasing the humidity level within the pocket space and decreasing the overall drying capacity of the papermaking machine.
The main object of the present invention is to provide a solution to the above-discussed ventilation problems so that offset pocket spaces of the drying section of a papermaking machine could be adequately ventilated.
In summary, the invention may be defined as a device for ventilating an offset pocket space located in a drying section of a papermaking machine by injecting air from at least one heated dry air supply. The offset pocket space is situated between three axially parallel drying cylinders over which consecutively runs a paper web, a first and a third of these cylinders being vertically spaced from a second of the cylinders. The paper web is pressed against the first and the third cylinder by a felt which further runs over a felt roll having a rotation axis parallel to that of the cylinders. The felt roll is disposed between the three cylinders in an offset position which is closer to the first cylinder than the third cylinder. The offset pocket space being delimited by a first cleft defined where the felt and the paper web separate, a first draw of the paper web from the first cleft to the second cylinder, a free portion of the second cylinder, a second draw of the paper web from the second cylinder to a second cleft defined where the felt and the paper web rejoin, and the felt between the first cleft and the second cleft. The device comprises:
a first air outlet where a first air stream is directed into a third cleft defined where the felt and the felt roll join; and
a second air outlet where a second air stream is directed in close proximity of the felt at a location which is upstream the third cleft, the second air stream being in a direction which is substantially opposite the running direction of the felt.
In use, the first air stream flows through the felt and lifts the paper web away from the felt to reach the offset pocket space, and the second air stream creates an air curtain to lower the pressure in a zone between the first and the second outlet so as to maintain the paper web on the felt as long as possible.
The present invention may also be defined as a method for ventilating an offset pocket space located in a drying section of a papermaking machine. The method comprises the steps of:
directing a first air stream from a first outlet into a third cleft defined where the felt and the felt roll join, the first air stream flowing through the felt and lifting the paper web away from the felt to reach the offset pocket space; and
directing a second air stream from a second outlet in close proximity of the felt, the second air stream being supplied at a location which is upstream the third cleft and in a direction which is substantially opposite the running direction of the felt so as to create an air curtain which lowers the pressure in a zone between the first and the second outlet.
Similarly, the present invention may be defined as a the method comprising the steps of:
producing an air stream directed into a third cleft defined where the felt and the felt roll join, the air stream flowing through the felt and then between the paper web and the felt in order to ventilate the offset pocket space; and
establishing a zone of reduced air pressure in close proximity of the felt at a location which is immediately upstream the third cleft so as to maintain the paper web on the felt as long as possible.
The objects, features and advantages will be apparent from the following detailed description of a preferred embodiment thereof, which proceeds with reference to the accompanying figures.